As is known, many devices based upon semiconductor technology are provided with cavities, such as chambers or channels, buried in a semiconductor body. In some cases, such as for example in pressure sensors, the cavities are sealed in a gas-tight way by deformable membranes. In this way, within the cavities themselves a constant reference pressure value is maintained at rest, whereas variations of the external pressure cause deformations of the membrane, which can be detected in various known ways. Typically, variations of the capacitive coupling are detected between the membrane and the underlying semiconductor body, or else piezoresistive circuit elements coupled in a Wheatstone-bridge configuration are used. In other devices, such as microfluidic devices that can be used for example as chemical microreactors or for the fabrication of ink-jet printer heads, the cavities comprise microchannels forming a microfluidic circuit. In this case, the microfluidic circuit is generally accessible from the outside through openings so as to receive the fluids necessary for operation of the device.
The formation of buried cavities or in any case of covered cavities in general raises some difficulties.
In order to overcome said difficulties, use of thermal processes of annealing has been proposed, which enable buried cavities of arbitrary shape to be obtained starting from trenches dug in a semiconductor body, causing a migration of the surrounding atoms. According to the technique described in EP-A-1 324 382, which is incorporated by reference, a semiconductor body, made, for example, of silicon, is initially anisotropically etched in order to dig adjacent trenches, close to one another and separated by diaphragms. The trenches are then closed without being filled by growing an epitaxial layer. Alternatively, instead of the trenches separated by diaphragms, it is possible to define a honeycomb structure of silicon pillars at a small distance from one another.
The epitaxial growth is performed in a deoxidizing environment rich in hydrogen, which remains trapped in the trenches (or in the interstices between the pillars) and is subsequently exploited in order to carry out an annealing step. In practice, the semiconductor body is heated to a pre-set temperature and for a pre-set time. Thanks to the deoxidizing atmosphere (rich in molecular hydrogen), the semiconductor material surrounding the cavity is subject to migration and tends to redistribute according to a minimum-energy configuration, maintaining in any case the order of the monocrystal. The diaphragms (or pillars) are thinned out progressively until they disappear altogether, and a single cavity is basically formed, closed by a portion of the epitaxial layer, which forms a suspended semiconductor membrane.
The known techniques present limits however. In fact, it has been noted that, during annealing, the hydrogen contained within the cavities tends to be dispersed through the epitaxial layer, which is thinner and partially permeable. Mere heating of the semiconductor body is not, therefore, as a rule sufficient to complete migration and obtain buried cavities of the desired shape. In order to prevent impoverishment of the internal deoxidizing atmosphere that may render the treatment ineffective, the annealing is likewise carried out in a controlled environment. Special machinery is hence often necessary, capable of controlling the environmental concentrations of gaseous species (in particular, hydrogen), such as for example epitaxial reactors or hydrogen ovens.